The present invention relates to a holographic memory apparatus that records and reproduces information to and from an optical information recording medium.
Today, an optical disc with the recording capacity of about 50 GB, to and from which information is written and read using a blue-violet semiconductor laser according to the Blu-ray Disc (BD) standard, is placed on the market. In the optical storage field, it is expected that a large-capacity optical disc with the recording capacity of 100 GB to 1 TB, comparable to that of an HDD (Hard Disk Drive), will be placed on the market.
However, the implementation of such a large-capacity optical disc requires a new storage technology different from the trend of the conventional high-density technology where much work has been done on a shorter wavelength and a higher NA objective lens.
Among the researches on the next-generation optical storage technology, attention is paid to the holographic recording technology for recording digital information using holography.
The holographic recording technology is a technology that superimposes a signal beam, which has page data information modulated two-dimensionally by a spatial light modulator, and a reference beam in a recording medium and, using the resulting interference fringe pattern, records the information by causing refractive index modulation in the recording medium.
When information is reproduced, the reference beam used for recording is irradiated on the recording medium in the same arrangement. Then, the hologram recorded in the recording medium acts as a diffraction grating to generate a diffracted light. This diffracted light is reproduced as the light, identical to the recorded signal beam, including the phase information.
The reproduced signal beam is detected quickly and two-dimensionally using an optical detector such as a CMOS or a CCD. In this way, holographic recording allows two-dimensional information to be recorded/reproduced at a time in one hologram, and multiple pieces of page data to be superimposed in the same position, thus recording/reproducing a large amount of information speedily and efficiently.
One of the holographic recording technologies is disclosed, for example, in JP-A-2004-272268 (Patent Document 1). This document describes the so-called angular multiplexing recording method in which a signal beam is focused on an optical information recording medium via a lens and, at the same time, the reference beam, which is a plane wave, is irradiated to cause interference with the signal beam for holographic recording and, while changing the incidence angle of the reference beam that falls on the optical recording medium, different page data is displayed on a spatial light modulator for multiple recording.
As the means for increasing the storage capacity of the holographic recording technology, a method is proposed in which multi-level phase information is added to the pixels of the signal beam. H. Noichi, H. Horimai, P. B. Lim, K. Watanabe and M. Inoue, “Collinear phase-lock holography for memories of the next generation” 12008 Digests, 42-43 (2008). (Non-Patent Document 1)